Selecting a Flight Level within the Quadrantal Rule

[paulbex]

Hi guys,

Sorry if this has been covered to death already and I've missed it, I searched for the answer within the forums but I couldn't find anything other than the basics of the quadrantal rule.

Basically on revision for a PPL exam I have this question (from airquiz.com):

You are intending to fly in accordance with the quadrantal rule on a magnetic track of 120. The QNH is 996 hPa. Which of the following is your correct flight level?

A FL 30
B FL 35
C FL 55
D FL 60

.... I know that a magnetic heading of 120 would require an odd flight level plus 500ft, so it's either B or C. But how do I know which one? What do I do with the QNH to calculate the correct FL?

Thanks very much in advance.

[paulbex]

Thank you sooooo much for answering this, the answer it gave me was indeed C.

I had that 510 figure and just didn't know what to do with it! :-)

[ddoth]

I wouldn't worry about the QNH. There are always those questions that add extra things to try and confuse you.
Over here, quadrant levels aren't required to be flown under 5000ft. Going off that bases I would say C.

Alternatively, the question is screwed up and either B or C should be 4500 / 6500.

[paulbex]

Brilliant, thank you guys I'll get doing some practice.

[2close]

You are intending to fly in accordance with the quadrantal rule on a magnetic track of 120. The QNH is 996 hPa. Which of the following is your correct flight level?

A FL 30
B FL 35
C FL 55
D FL 60

NewMan123 has it spot on but just to clarify it from an instructional point of view.

This question requires you to know the difference between an Indicated Altitude (above mean sea level and based on QNH) and a Pressure Altitude (Flight Level based on 1013 hPa).

The quadrantal rule requires ODD + 500’ to be flown on magnetic tracks between 090°M – 179°M.

This immediately restricts the answers to B (FL35) and C (FL55).

For the purpose of this examination, the Transition Altitude in the UK is 3,000’, therefore the lowest Flight Level is FL35.

The purpose of the QNH in this question is to find out whether or not you are above the Transition Altitude. A QNH LESS than 1013 hPA tells me immediately that the Indicated Altitude will be LESS than the Pressure Altitude.

First of all, find out the pressure correction.

This is my method for calculating an Indicated Altitude from a Pressure Altitude.

Pressure Correction = (QNH-1013) x 30’
Pressure Correction = (996-1013) x 30’
Pressure Correction = -17 x 30’
Pressure Correction = - 510’

That looks immediately suspicious for FL35 (Pressure Altitude of 3,500’) being correct but let’s check.

Indicated Altitude = Pressure Altitude +/_ Pressure Correction
Indicated Altitude = 3,500’ – 510’
Indicated Altitude = 2,990’

This is below the Transition Altitude and therefore you would not fly on a Flight Level.

This only leaves C (FL55) as the only possible answer.

Hope that helps.

[BillieBob]

I wouldn't worry about the QNH.

I wouldn't worry too much about quadrantals either, they won't exist soon.

[2close]

Very true and the Transition Altitude in many areas of the UK is now 6,000' and not 3,000'.

.......and 'Selective Availability' has not been a function of the NAVSTAR/GPS system since May 2000 but there are still questions on it in ATPL Radio Navigation, as there are on many defunct and out-of-date areas.

I could quote many examples but the point I'm really trying to make is that the OP stated he was preparing for a PPL exam - it takes the 'competent authorities' a while to catch up with the real world so it pays to stay on top of apparently redundant topics until you know for certain that they're not going to be in the exams.

[172510]

You can only start using FL when you're at least 3000 ft on the QNH.

If you were to fly FL 35 you would be flying an altitude of 2990 ft.

So I believe it has to be FL 55.

Even if your altimeter is set to 1013 and reads 3500, you are legal, whatever your track is, if the QNH of the day is low enough to keep your altitude below 3000ft.